The most common example of this type of attack is the "many laughs" attack (sometimes called the 'billion laughs' attack). For example: ]> &lol9; This is well formed and valid XML according to the DTD. Each entity increases the number entities by a factor of 10. The line of XML containing lol9; expands out exponentially to a message with 10^9 entities. A small message of a few KBs in size can easily be expanded into a few GB of memory in the parser. By including 3 more entities similar to the lol9 entity in the above code to the DTD, the program could expand out over a TB as there will now be 10^12 entities. Depending on the robustness of the target machine, this can lead to resource depletion, application crash, or even the execution of arbitrary code through a buffer overflow.
This example is similar, but uses YAML. This was used to attack Kubernetes [REF-686] a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"] b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a] c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b] d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c] e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d] f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e] g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f] h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g] i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]